Fuel assemblies and control rods are positioned in the core of the nuclear boiling water reactor, BWR. The channel boxes of the fuel assemblies in the BWR usually consist of a corrosion resistant material with a low neutron absorption capacity, such as a zirconium based alloy.
The environment in the core of a BWR is demanding for the components positioned therein. The environment is for example highly oxidative. One of the consequences of this demanding environment inside the core of a BWR is that the channel box of the fuel assemblies may be distorted. The channel box may for example bulge or bow. Channel box bow is due to elongation of one channel box side relative the opposite channel box side. Channel box bow is known to arise for different reasons, e.g. initial manufacturing, residual stress relaxation under irradiation, differential irradiation growth and hydration as a consequence of shadow corrosion.
The problem of shadow corrosion on components comprising zirconium based alloys in the core of a BWR has been known for a long time. Shadow corrosion is a local corrosion enhancement and can appear on a zirconium based alloy component when the component is in close contact with another metal. Referring to the above, shadow corrosion on the outer side of a channel box can occur when a control rod blade is inserted next to the channel box, i.e. when the channel box consisting of a zirconium based alloy is in close contact with a control rod blade usually having an outer surface of stainless steel.
The degree of shadow corrosion depends on the distance between the two components. Direct contact gives most shadow corrosion and at a distance of about 5 mm it effectively disappears. Most of the effect has disappeared already at a distance of 2 mm.
Shadow corrosion early in the life of a fuel assembly, i.e. shadow corrosion on the fuel assembly due to an inserted control rod next to the fuel assembly during the first several months of operation, is generally believed to drive the problem of enhanced channel bow of the channel boxes in a BWR. The shadow corrosion can result in increased absorbed hydrogen-induced growth of the outer side of the channel box being closest to the control rod. The increased absorbed hydrogen-induced growth can lead to bowing of the channel box towards the control rod late in the life of the fuel assembly. The bow of the channel box towards the control rod may for example slow down or stop the control rod insertion in an emergency situation, or may lead to channel box-control rod interference, which may for example cause the fuel assemblies to lift due to friction when the control rods are inserted into the core.
The control rods are tested periodically with regard to proper functioning and interference. In case some interference is noted or suspected the testing intervals are shortened. With severe interference the control rod is parked in the safe position, i.e. fully inserted and disarmed. In either case the intended operation is disturbed as an ultimate result of the shadow corrosion.
Studies have shown that shadow corrosion strongly depends on the distance between the zirconium based alloy component and the component comprising another metal. The occurrence of shadow corrosion is therefore most significant in the case of a large control rod blade and a small distance between the control rod blade and the channel box.
The European patent application EP 0 986 069 describes a device and method for preventing shadow corrosion. The device comprises means arranged to electrically insulate at least a part of a second component from a first component in order to avoid shadow-corrosion on the first component.